1. Field of Art
This invention relates generally to crosslinkable polyethylene resin compositions. More particularly the invention relates to polyethylene resin compositions the resins of which are crosslinkable by exposure to water, each composition comprising a random copolymer of ethylene and an ethylenically unsaturated silane compound having a hydrolyzable silane group as a crosslinkable group and a crosslinking reaction catalyst.
The procedure of crosslinking low-density polyethylenes and other polyethylene resins thereby to improve their mechanical strength, heat resistance, and other properties, and various crosslinking techniques for this purpose are known.
2. Prior Art
One known crosslinking technique of this character comprises adding an organic peroxide as a crosslinking agent to a polyethylene and heating these materials to a high temperature to decompose the peroxide and thereby to initiate the crosslinking reaction. In this case, however, since crosslinking due to the decomposition of the peroxide is carried out prior to the step of forming the polyethylene, the quality of the formed product is frequently defective, and, in extreme cases, the forming cannot be carried out satisfactorily.
If the crosslinking is to be carried out after the forming step, a high-temperature effective peroxide which can withstand the forming temperature must be used. In order to decompose a peroxide of such a character, the formed article must be heated to a temperature higher than the forming temperature. As a result, variations due to softening of the formed article will occur in the crosslinking step, and the quality of the formed product will still become defective.
As an example of a polyethylene which is not accompanied by such problems encountered in chemical crosslinking, there is a silicone-grafted polyethylene (as disclosed, for example, in Japanese Patent Publication No. 1711/1973, Patent Laid Open No. 8389/1972, Patent Laid Open No. 138042/1975, and Patent Laid Open No. 9073/1977). When a silicone-grafted polyethylene is exposed to water, its silicone part is hydrolyzed, and the crosslinking reaction progresses. Since this hydrolysis occurs at a relatively low temperature, the large-scale equipment required for chemical crosslinking becomes unnecessary. For this reason applications to crosslinked molded products are being studied.
In this case, however, a plurality of process steps for producing the base polyethylene and for grafting the silicone onto the base polyethylene are necessary. Moreover, a polyethylene article produced and crosslinked in this manner, such as, for example, a sheet, is unsatisfactory with respect to its odor, mechanical strength, heat resistance, and heat welding property.
On one hand, a method of causing crosslinking of copolymers of ethylene and a vinyl alkoxysilane by heating or by mechanical working (as disclosed in the specifications of U.S. Pat. Nos. 3,225,018 and 3,392,156) is known. This method is advantageous relative to the above mentioned grafting method in that a crosslinkable polyethylene is obtained with a single process step, that is, with only the copolymerization step. In this case, however, since the crosslinking unavoidably must be carried out prior to the forming process, the same problems as in the case of the above mentioned chemical crosslinking are encountered. Moreover, the mechanical strength of a formed article produced by forming after the crosslinking reaction is not satisfactory.